The present invention relates to radio transmitters and receivers, and to oscillators for use therein. In particular, the present invention, in its various aspects, addresses problems that arise in connection with designing and fabricating radio circuits on integrated circuit chips, and particularly on CMOS integrated circuit chips.
FIG. 1 discloses a known technique for reception of Amplitude-Shift-Keyed (ASK) radio signals using a local oscillator having quadrature phase components, including systems for direct conversion to baseband. In the receiver 10, a modulated carrier signal, x(t), is provided on input line 12 to mixers 14 and 16. The mixers multiply the modulated carrier x(t) by two local oscillator signal components in phase quadrature, to thereby detect in-phase and quadrature-phase components of the received signal at the intermediate frequency (IF). The in-phase and quadrature-phase components of the received signal at IF are isolated from the mixers"" (14 and 16) outputs by low-pass filters 18 and 20. The instantaneous signal energies of the in-phase and quadrature-phase components of the received signal at IF are detected by energy detectors 22 and 24. The detected in-phase and quadrature-phase instantaneous signal energy outputs are summed in combiner 26 to produce an output signal J representative of the instantaneous baseband signal energy, which is independent of the phase relation between the local oscillator and the transmitted carrier. In amplitude-shift-keying systems, the instantaneous baseband signal energy is also representative of the original baseband signal which modulates the transmitted carrier. Hence, the demodulation process is achieved.
One difficulty associated with using a detector having the arrangement of FIG. 1 is the requirement of providing local oscillator signal components having quadrature-phase relation.
FIG. 2 illustrates one possible arrangement for providing quadrature phase components of a local oscillator in an integrated circuit configuration. The oscillator 30 of FIG. 2 comprises a four-stage differential ring oscillator having differential amplifiers 32, 34, 36 and 38. The circuit of FIG. 2 oscillates with period equal to the total time delay of the four differential amplifiers. The time delay of each differential amplifier can be varied, thereby to change the frequency of oscillation, by providing a voltage control on lead 40 which changes the propagation delay through each of the differential amplifiers. Since the total phase delay around the loop in the differential ring oscillator 30 is 360xc2x0, quadrature local oscillator signal components can easily be obtained at the outputs of adjacent differential amplifiers, such as amplifiers 32, 34, as shown in FIG. 2.
The oscillator of FIG. 2 can be difficult to implement where the required oscillation frequency is relatively high, such as 2.4 GHz allocated to the unlicensed ISM (Industrial, Scientific and Medical) frequency band. In attempting to implement differential ring oscillator 30 using CMOS technology, it has been found that the maximum frequency available with four stages is about 1.9 GHZ. Differential ring oscillator 30 can be implemented with three differential amplifier stages, but such arrangement will not provide the quadrature or orthogonal phase local oscillator components required in the receiver 10 of FIG. 1.
FIG. 3 is a graph showing the relation of the control voltage V for the oscillator 30 to the operating frequency. Typically, small variations in the control voltage xcex94V, can produce significant variations in the frequency of the oscillator output. Where the control voltage has a periodic component, as may arise by reason of leakage of a reference oscillator signal in a phase locked loop, the resulting variation in control voltage V will produce undesired FM sidebands in the output signal of oscillator 30.
FIG. 8 discloses a classic quadrature direct-conversion technique for reception of frequency modulated (FM) radio signals. In the receiver 120, a modulated carrier signal, x(t), is provided on input line 122 to mixers 124 and 126. The mixers multiply x(t) by two local oscillator signal components in phase quadrature, to thereby detect in-phase and quadrature-phase components of the received signal at the intermediate frequency (IF). The in-phase and quadrature-phase components of the received signal at IF are isolated from the mixers"" (124 and 126) outputs by low-pass filters 128 and 130. The frequency modulation of the in-phase and quadrature-phase components of the received signal at IF are detected by differentiators 132 and 134. The detected in-phase and quadrature-phase frequency modulation outputs are multiplied in mixers 136 and 138 by the quadrature-phase and in-phase components, respectively, of the low-pass filtered received signal at IF and then are summed in combiner 140 to produce an output signal y(t) representative of the demodulated frequency deviation. The in-phase and quadrature-phase components of the received signal at IF are also each squared in detectors 142 and 144 and then summed in combiner 146 to produce an output signal J(t) representative of the instantaneous received signal energy, which is independent of the phase relation between the local oscillator and the transmitted carrier.
It is an object of the present invention to provide a radio frequency receiver which can be implemented in an integrated circuit without suffering from the disadvantages of the art.
It is a farther object of the invention to provide an integrated circuit oscillator having an improved suppression of undesired FM sidebands in the oscillator output.
In accordance with the invention, there is provided an amplitude-modulated signal receiver including a local oscillator providing at least three equally-phased local oscillator signal components. The local oscillator signal components are mixed with a modulated carrier signal and the resultants are provided to corresponding signal detectors. The output of the signal detectors are combined to provide a received signal,
In accordance with the present invention the local oscillator may be implemented as a voltage-controlled oscillator in a phased-locked loop. The voltage-controlled oscillator has a frequency which is controlled by first and second current control devices, including coarse and fine current control devices. Preferably the coarse current control device is controlled by a signal which is selected in discrete steps, while the fine current control device is controlled by an analog signal.
The phase locked loop may include signal comparators for comparing a phase error signal to upper and lower values to implement adjustments of the coarse control signal in discrete steps. The phase error signal is used for controlling the fine current control device.
In accordance with the invention, there is also provided a frequency-modulated signal receiver including a local oscillator having at least three equally phased components. Each of the phase components of the local oscillator is mixed with the received signal and the outputs of all but one of the mixers are additively combined. A discriminator responsive to the outputs of the one uncombined mixer and the combiner provides an output signal representative of the demodulated frequency modulation. Preferably, the discriminator includes a first differentiator responsive to the one uncombined mixer output, a second differentiator responsive to the combiner output, a first mixer responsive to the first differentiator output and the combiner output, a second mixer responsive to the second differentiator output and the one uncombined mixer output and a second combiner for combining the outputs of the first and second mixers.
In accordance with the invention, there is also provided a receiver for receiving phase modulated signals including the frequency modulated signal receiver and a device for integrating the signal representative of frequency modulation to derive a signal representative of phase modulation.
Finally, in accordance with the invention, there is also provided a local oscillator which provides first and second local oscillator component signals in phase quadrature. The local oscillator includes an oscillator providing at least three equally-phased component signals and a combiner for combining all but one of the component signals such that the output of the combiner includes a local oscillator signal in phase quadrature with the one uncombined component signal.
For a better understanding of the present invention, together with other and further objects, reference is made to the following description, taken in conjunction with the accompanying drawings, and its scope will be pointed out in the appended claims.